Tokyo Bay Aqua-Line

A 15.1 km sea-crossing road combining a 9.5 km undersea tunnel with a 4.4 km offshore bridge, linking Kawasaki in Kanagawa with Kisarazu in Chiba. Opened in December 1997.

Use

Bridge

Area

Kawasaki / Kisarazu

Completed

1997

Developer

Japan Highway Public Corporation (now NEXCO East Japan)

A Single Line Across Tokyo Bay

The Tokyo Bay Aqua-Line stretches approximately 15.1 kilometres across Tokyo Bay, connecting Kawasaki in Kanagawa Prefecture with Kisarazu in Chiba Prefecture. When it opened on December 18, 1997, a crossing that once required a 40-minute ferry ride shrank to roughly 15 minutes by car.

The Aqua-Line consists of two distinct structures. On the Kawasaki side, the Aqua-Tunnel plunges approximately 9.5 kilometres beneath the seabed—among the longest undersea road tunnels in the world at the time of its completion. On the Kisarazu side, the Aqua-Bridge carries traffic along a 4.4-kilometre viaduct riding low over the water’s surface. The two structures meet at an artificial island, Umi-Hotaru, which serves as a rest area and offers panoramic views of the bay on clear days.

Engineering the Seabed

The concept of crossing Tokyo Bay dates to the 1970s, but the staggering costs kept the project on hold for decades. Construction finally began in the late 1980s, and the total project took roughly eight years and seven months to complete. The total construction cost reached approximately 1.44 trillion yen.

The most formidable challenge was laying the tunnel through soft seabed soil. Workers used a sunken-tube method, sinking large steel caissons into the bay floor and connecting them one by one, all while contending with Tokyo Bay’s strong tidal currents. The final caisson was joined in August 1996, completing the tunnel before the full route opened the following year.

Summary

The Tokyo Bay Aqua-Line—opened in 1997—combines a roughly 9.5 km undersea tunnel with a 4.4 km offshore bridge to span approximately 15.1 kilometres of Tokyo Bay. The mid-point rest area at Umi-Hotaru has become a destination in its own right, drawing visitors to its sweeping views of the bay skyline. For anyone driving the route, the Aqua-Line offers a rare chance to experience the engineering scale that holds this busy stretch of sea together.